The present invention relates to a thin film electroluminescent (TFEL) display panel and more particularly, to a thin-film electroluminescent display panel shielded by a pair of glass substrates with a protective material disposed therebetween.
For general background information on TFEL panels, see the "EL Glass Catalog and Design Handbook," Planar Systems, Inc., Beaverton, Oreg. 97006, the contents of which, to the extent necessary, are hereby incorporated herein by reference.
A conventional TFEL display panel is illustrated in FIG. 1, wherein the panel comprises a first transparent glass substrate a plurality of transparent electrodes 2 made of In.sub.2 O.sub.3 or SnO.sub.2, and the like, a first dielectric layer 3, an electroluminescent (EL) thin film 4, a second dielectric layer 5, a plurality of counter-electrodes 6 made of for example Al, spacers 10, and a counter-substrate or cover plate 11, which may be made of glass. See, for example, U.S. Pat. No. 4,213,074 to Kawaguchi et al.
As illustrated, the transparent electrodes 2 are arranged on the glass substrate 1 in parallel with each other. The counter-electrodes 6 are arranged so that they cross at a right angle relative to the transparent electrode 2 in a plane view. The cross points between each of the transparent electrodes 2 and the counter-electrodes 6 create a picture element (pixels) i.e., the image forming portion of the TFEL panel. A power source (not shown) is applied to the transparent electrode 2 and the counter-electrode 6.
The first dielectric layer 3 may comprise Y.sub.2 O.sub.3, TiO.sub.2, Al.sub.2 O.sub.3, Si.sub.3 N.sub.4, SiO.sub.2, and the like, which may be deposited for example by a sputtering technique or by electron beam evaporation. The EL thin film 4 may be made for example, from a ZnS thin film doped with an impurity, for example manganese. The second dielectric layer 5 generally comprises a material similar to that of the first dielectric layer 3.
The TFEL panel is generally provided with a sealing structure for the EL composite member which comprises the first and second dielectric layers 3, 5 and the thin EL film 4. The cover plate 11, together with the transparent glass substrate 1, provide the basic structure for sealing the EL unit. The cover plate 11 need not be transparent because viewing may be conducted through the transparent glass substrate One or more spacers 10 may be employed for positioning the cover plate 11. An adhesive 12 is coated for bonding the transparent glass substrate 1, the spacer 10, and the cover plate 11.
An adhesive 12 is generally employed, which may be an epoxy resin or the like. Lead terminals 15 of the transparent electrodes 2 and the counter-electrodes 6 may be formed on the transparent glass substrate 1 and extended toward the cavity. A control circuit (not shown) is coupled to the lead terminals 15 to apply the power to the EL unit.
A protective substance 13 may be added to the cavity defined by the two plates 1 and A protective substance 13 functions to preserve the TFEL panel, especially the EL unit. The protective substance may be a gas or a liquid, but liquids are preferred. See, for example, U.S. Pat. No. 3,330,982 to Dickson, and U.S. Pat. No. 4,447,757 to Kawaguchi et al. Typical protective gases include inert gases such as nitrogen, argon, and the like. Typical protective liquids include silicon oils or greases.
A spacer 10 may be employed, and it may be formed from an insulating plastic sheet made of for example, a polyacetal resin or a polyimide resin, or a silicon rubber, or a glass plate. Finally, at least one fill hole 14 is generally provided, for the introduction of the protective substance 13.
If desired, a dye material or other color agent may be added to the protective substance in the TFEL panel to provide a background which can aid in the display characteristics of the panel.
TFEL panels of the type illustrated in FIG. 1 are very susceptible to moisture and therefore must be properly protected.
As discussed previously, the usual way to protect these panels is to cover the EL film with a transparent glass cover plate. This cover plate may be attached to the substrate plate by a perimeter seal, which is usually prepared from a moisture proof elastomer. In some cases glass frit seals have been applied, although such seals are impractical due to temperature limitations and chemical reactions with fritted-over contact leads.
TFEL panels that are merely protected by a cover plate also have a certain volume of air trapped in between which can vary in terms of humidity. Any water vapor coming into contact with the EL film will, over a period of time, interfere with the performance of the EL film, as the ZnS active layer is very hygroscopic.
The above-described sealing process could be conducted in either a dry nitrogen or argon atmosphere, or there could be incorporated into the EL unit a small amount of desiccant, such as silica gel or phosphorous pentoxide. See, for example, U.S. Pat. No. 4,357,557 to Inohara et al. However such procedures are not believed to readily lend themselves to mass production or to working with large TFEL panels.
One approach at providing a complete seal for an EL panel (to keep moisture out, protective substances in) involved sealing the two panels with a perimeter seal and leaving a small fill hole in the cover plate (perpendicular to the plate area). The panel was then immersed in an oil bath and the entire assembly placed in a vacuum oven. See for example U.S. Pat. No. 4,213,074 to Kawaguchi et al.
Any air trapped between the two panels escaped during the pumpdown process. After this the system was back filled with dry nitrogen which forced the oil through the hole and filled the volume between the two plates. While this method worked in principal, drawbacks included frequent explosions during pumpdown since the air above the oil bath was removed much faster than the air volume between the plates.
Another disadvantage with this method was the fact that there was generally a small bubble left after oil filling which could not be eliminated.
The most difficult problem involved the inability to seal the fill hole, due to the fact that it was covered with oil. Also the clean up of the panel was messy and time consuming.
An improvement in the above-described method was achieved by introducing small fill tubes attached to the cover plates. These tubes were connected to small "Tygon" hoses which were hung into the protective oil. The basic pumping procedure remained the same, and this modification eliminated the messiness and the wetting of the tip off area. However, the problems of explosions and residual volume were not solved.
Further progress in the introduction of protective liquids into TFEL panels was made by abandoning the vacuum method. Instead a syringe with two check valves was used to fill the panels in a relatively short time. The explosion problem, the residual "bubble", the oil wetting and the fill time factor were solved.
However, with the new modifications came geometric restrictions as to the thickness of the panel assembly due to the bezel construction. Thus, the fill tubes could no longer be mounted perpendicularly into the cover plate. This problem was solved by drilling a hole or channel parallel to the plate plane into the cover plate. A TFEL panel illustrating this type of fill hole 100 is shown in FIG. 2.
After filling the FIG. 2 type TFEL panel with a protective liquid, the fill tubes were crimped and tucked in the corner space and sealed over with epoxy. This solution seemed to be ideal. The panel kept its thickness and the oil fill technique was straight forward. The only real difficulty was in drilling the hole, but in most cases, it could be done.
In general, the thickness of the cover glass plates for TFEL panels prepared using the parallel hole filling method ranged from about 0.043 in. to 0.065 in. However, drilling a 0.040 in. diameter channel into the plate was almost impossible in the case of the thin glass, but having no protruding parts at the rear side of the panel seemed to justify this method of construction for a long time.
After a while, it was observed that a good number of parallel fill hole TFEL panels, at least about 20%, seemed to leak after a couple of days, especially after being handled frequently. Upon visual inspection, no direct leak along the seal areas could be found. After careful inspection under a microscope, hairline cracks were observed extending perpendicularly from the fill tube channel. While not wishing to be bound by theory, it is believed that the hydraulic forces during oil filling were strong enough to cause this breakage.
Clearly, the fill hole construction detail of TFEL panels had to be revised again.
The present invention is directed to this latest development in fill hole construction detail, solving the problems of the previously described designs, and resulting in a TFEL panel of exceptional strength and durability.